The present invention relates to a sintered alloy which exhibits superior wear resistance and to a process of manufacture therefor, and more particularly, relates to a technique suitable for use for valve seats in internal combustion engines.
Recently, with the increasing performance of automobile engines, operating conditions have become much more severe. The valve seats used for such engines are also inevitably required to withstand more severe environments than before. To meet such requirements, the present applicant previously proposed several sintered alloys having superior wear resistance as disclosed in, for example, Japanese Examined Patent Publications (KOKOKU) Nos. 17968/74, 36242/80, 56547/82, 55593/93, and 98985/95.
Of all the proposed sintered alloys having superior wear resistance, the sintered alloy disclosed in Japanese Examined Patent Publication (KOKOKU) No. 55593/9395 is particularly improved in wear resistance. The sintering alloy exhibits a metallographic structure in which diffusing phase diffused Co is surrounded by a hard phase consisting of Mo silicide in a matrix structure, and superior wear resistance is obtained by the presence of the hard phase. A matrix is disclosed in Japanese Examined Patent Publication (KOKOKU) No. 36242/80. A sintered alloy having superior wear resistance disclosed in Japanese Examined Patent Publication (KOKOKU) No. 98985/95 is an improvement of the alloy disclosed in Japanese Patent Examined Publication (KOKOKU) No. 55593/93. By including Ni in an amount of 5 to 27% by weight in the alloy of Publication (KOKOKU) No. 55593/93, the matrix structure is strengthened, thereby further improving wear resistance.
However, since these alloys use expensive materials such as Co, they may not meet the demands for recent cost-performance. That is to say, the development of automobiles is recently directed not only to higher performance but also to lower cost from an economic point of view. Therefore, the present applicant proposed a sintered alloy having superior wear resistance which can yield the required wear resistance using inexpensive materials in Japanese Examined Patent Publication (KOKOKU) No. 195012/97. In this proposal, by using a powder which partially diffuses each powder of Ni, Cu, and Mo into Fe powder, as a matrix forming powder, the matrix is strengthened, and by dispersing the hard phase primarily consisting of Cr carbide into this matrix structure, the required wear resistance and mechanical strength are obtained without using expensive materials such as Co.
However, the demands on cost-performance become more severe every year, and a sintered alloy having superior wear resistance for the valve seat, which is less expensive than the above-proposed sintered alloy having superior wear resistance, is further demanded. Therefore, since expensive materials such as Mo are used in the above-proposed sintered alloy having superior wear resistance, it seems that there is room for further improvement concerning the use of materials.
At present, the conditions for operation are even further increased in severity as the performance of automobile engines continues to improve, and a material, which is superior in wear resistance and in strength to the above-mentioned sintered alloys, is demanded.